Sounds like a pop-up to me or some delightful piece of spyware or quite possible a dialer. They are nasty. They go away if you download FIREFOX FROM
WWW.MOZILLA.ORG and stop using Internet Explorer.
i would also go
here
and download ad-aware which you will then run to find the 40 or so pieces of critical level spyware you have installed through your naive use of the evil Internet Explorer
Hopefully this finds dialers as well, but if you use normal dial-up I'd check your modem settings - or at the least phone BT and check your current phone bill
